Regulated voltage supply for screen grid



R. K. DAVIS REGULATED VOLTAGE SUPPLY FOR SCREEN GRID Sept. 23, 1958 2,853,611

Filed De 5. 1956 INVENTOR.

ROBE/PT K. DAV/S United States Patent REGULATED VOLTAGE SUPPLY FOR SCREEN GRID Robert K. Davis, Indianapolis, Ind.

Application December 5, 1956, Serial No. 626,550 3 Claims. c1. 250-27 (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties-thereon or therefor.

In accordance with this invention, a screen-grid voltage regulator comprises a grid-controlled, space-discharge device having its cathode coupled to a screen-grid, its plate coupled to a source of unidirectional potential, and its control grid coupled to a source of reference voltage.

The function of the screen-grid in space-discharge devices is to provide an electrostatic shield between the electrodes which it separates. As such, the screen-grid effectively minimizes interelectrod'e capacitance and substantially eliminates the influence of voltage variations at the output element on output current. Inasmuch as the aforementioned function requires that the screen-grid be biased to a potential higher than that of the electron-discharge electrode, it accelerates the electrons present in the interelectrode space. Although the structure of the screen grids is such that most of the accelerated electrons will pass through andthence proceed to the output electrode, some of therninevitably strike the grid and thereby pro duce current in the screen-grid circuit.

Many conventional potential-supply circuits for screen grids have internal impedances of fixed characteristics. Therefore, fluctuations in the magnitude of screen-grid current necessarily produce corresponding variations in the potential supplied to the screen grid, thereby causing the total volume of electrons escaping from the spacecharge region surrounding the electron-discharge electrode (equal to screen-grid plus output current) to increase and decrease accordingly,

The effect of variations in screen-grid potential is analogous to the effect of voltage changes on the output element of a three-electrode, space-discharge tube such as, for example, a conventional triode. It is wellv known, of course, that an increase in the plate current of a conventional plate-loaded triode is produced when the potential on its control grid becomes higher. This increased plate current results in a loss of effective voltage at the plate which, in turn, reduces the increase in plate current that otherwise would occur. The loss of plate current which occurs as the effective plate voltage of a triode is reduced by the voltage drop across its load or, in the case of a screen-grid device, when the effective screen-grid potential is reduced by the voltage drop across the impedance in its potential-supply circuit, results in a corresponding loss in amplification and increases distortion, especially harmonic distortion.

In embodiments of this invention, the potential on a screen-grid is rendered substantially independent of a screen-grid current by a voltage regulating circuit comprising a grid-controlled, space-discharge device coupled as a cathode follower between the source of supply potential and the screen-grid; a source of reference voltage is applied to the control grid of the space-discharge device. The function of the space-discharge device is to pro vide an inversely varying impedance in the screen-grid supply circuit, an impedance which decreases with increasing and increases with decreasing screen-grid current, such that any tendency of the screen-grid potential to change with respect to the reference potential applied to the control grid is compensated. As a result, the potential difference between the source of reference potential and the screen-grid is stabilized.

A first exemplary embodiment of the invention may comprise a first space-discharge device having a screen grid, and a second, grid-controlled, space-discharge device such as a triode having its cathode coupled to the screen grid of the first device, its plate coupled to a source of plate potential, and its control grid coupled to a source of reference potential which, for example, may be derived from a potentiometer coupled between a platepotential source and a ground source of constant potential.

Inasmuch as the triode coupled in the screen-grid circuit functions as a conventional cathode follower, the potential on its cathode tends to change in proportion to potential changes on its control grid. Conversely, where a constant control grid potential is maintained, the potential on the cathode will be stabilized. Thus, whenever,

A screen-grid potential is unchanged, variations in current at the output. of the screen-grid device more accurately represent the variations of the input signal. It should be noticed, of course, that any fluctuation in the controlgrid potential of the triode regulator tube will be reflected in a corresponding change in the screen-grid potential. This is not a disadvantage in most applications, especially those where the plate supply potential is stabilized. Moreover, as described hereinafter, this fact is utilized to advantage in other embodiments of the invention.

A second exemplary embodiment of the invention may comprise a first space-discharge device having a screengrid, and a second, grid-controlled, space-discharge device such as. a triode having its cathode coupled to the screen-grid of the first device, its plate coupled to a source of plate-supply potential, and its control grid coupled to a source of reference potential which, for example, may be derived from a voltage divider coupled between a source of plate supply potential and the cathode circuit of the first device.

As explained in connection with the first exemplary embodiment, the fact that the regulator triode functions as a cathode follower means that any change in reference potential applied to its control grid will be reflected in a corresponding change in the potential at its cathode. Accordingly, any potential change occurring at the oathode of the first device will appear also at the screen-grid.

To avoid degenerative efiects, suitable bypass capacitors maybe used where required. For example, where the control grid of the regulator triode is coupled via a suitable bypass capacitor to the cathode of the first space-discharge device, the screen-grid potential follows closely the potential variations occurring at the cathode, such that the potential difference between the screengrid and the cathode remains substantially constant for unattenuated-frequencies of cathode potential variation. Where the amplitude of the input signal becomes high enough to increase the average current in the screen circuit, the regulator triode effects compensation of screengrid potential in the same way that regulation is elfected in the first exemplary embodiment.

A third exemplary embodiment of the invention may comprise two screen-grid devices coupled in push-pull relation and having screen potential regulator circuits such ,as those described in connection with the second exemplary embodiment.

Heretofore, screen supply potentials have been derived from separate supply sources having low internal impedances, through dropping resistors from output potential supply sources, and from voltage dividers having low impedance bleeder paths coupled between the screen-grid and a source of constant potential. These methods of regulation are satisfactory for applications Where moderate degrees of screen-grid potential regulation are acceptable. Closer regulation of the screen-grid potential may be obtained from the use of gas regulator tubes. However, this method does not provide flexibility in the design choice of screen-grid potential. Moreover, current drain through voltage regulator tubes is high, thereby making it necessary to provide a power supply having a higher current rating.

Close regulation of screen-grid potential is especially desirable where a screen-grid device is utilized for power amplification. In such circuits, screen-grid devices necessarily must pass large currents in response to comparatively high input signals. The increasing screen current resulting from an increasing input signal produces a greater voltage drop across the impedance of the screen circuit, thereby reducing the effective screen potential and total space current, and resulting in increased power loss and amplitude distortion, especially third harmonic distortion.

This invention averts losses in screen potential of the kind described and thereby makes it possible substantially to reduce power loss and to minimize spurious harmonics present in the output of power amplifiers.

From the foregoing introductory statements, it should be apparent that it is among the objects of the invention to provide:

(1) Means for regulating the potential applied to the screen-grid of a space-discharge device.

(2) Means for regulating the potential applied to the screen-grid of a space-discharge device in accordance with a changing or fixed reference potential.

(3) Means for stabilizing the potential difference between the electron-discharge element and the screen-grid of space-discharge devices, notwithstanding fluctuations in the potential of the discharge element.

(4) Means for minimizing the distortion of output components representative of the input signals applied to screen-grid devices.

(5) An amplifier wherein harmonic distortion attributable to screen-grid voltage variations substantially is eliminated.

(6) A power amplifier wherein third harmonic distortion attributable to screen-grid voltage variations substantially is eliminated.

(7) An amplifier wherein losses in output power attributable to losses in screen-grid voltage are prevented.

(8) A variable impedance means regulating the potential of the screen-grid of a space-discharge electron device.

(9) Variable impedance means for maintaining the potential applied to the screen-grid of a space-discharge, electron device at a constant magnitude notwithstanding fluctuations of screen-grid current. V

(10) Impedance means varying automatically to compensate for fluctuations in the screen-grid current of a space-discharge electron device, such that the potential applied to the screen-grid is stabilized at a pre-established reference level.

(11) Means of superior economy and engineering simplicity for elfectuating any of the foregoing objects.

The foregoing summary of the invention, statement of the problem evoking its origination, and enumeration of 4 its objects are intended merely to facilitate the development of an understanding and appreciation of its principal features, not to restrict its scope. It is probable that additional objects and features of the invention will become apparent after reference to the following detailed description made in conjunction with the accompanying illustrations wherein:

Fig. lrepresents a first embodiment of the invention wherein the reference potential of the screen-grid regulator tube is derived from a potentiometer coupled between a source of output potential and a ground source of constantpotential,

Fig; 2 represents a second embodiment of the invention wherein the reference potential for the screen-grid regulator tube is derived from a voltage divider coupled be tween the output voltage supply source and the cathode of the screen-grid device, and

Fig. 3 represents the invention embodied in a pushpull amplifier circuit.

In Fig. l the invention is represented as embodied in a pentode amplifier circuit comprising a conventional pentode 1 and a regulator triode 2 having its cathode 3 coupled directly to the screen-grid 4 of the pentode, its control grid 5 coupled directly to the pickolf arm of potentiometer 7, and its plate 9 coupled directly to a source of B+ potential. The resistive element of potentiometer 7 is coupled between the source B+ potential and theground source of constant potential. The plate 11 of the pentode is coupled via load resistor 13 to the source of B+ potential, and its suppressor grid 15 is coupled directly to the cathode 17. An input signal e may be applied to the control grid 19, and an output signal e may be taken from the plate circuit.

The cathode 17 of the pentode is coupled directly to the ground source of constant potential.

When the magnitude of e; increases, the space current discharged from cathode 17 also increases. Inasmuch as the ratio of current in screen-grid 4 to the current absorbed by the plate 11 remains substantially constant over a considerable range of space current variation, the volume of electrons received by screengrid.4 also increases, thereby reducing the potential of cathode 3 of the triode regulator tube 2. The reduction in the potential of the cathode 3 decreases the potential difference between control grid 5 and cathode 3. As a result, the tendency of the cathode 3 to become more negative is counteracted effectively by a corresponding increase in the flow of current'through the regulator triode 2. As a result, the potential on screen-grid 4 remains substantially constant notwithstanding fluctuations in space current through pentode 1.

By adjusting the position of the pickoif arm of potentiometer 7, the reference potential on control grid 5 of triode regulator tube 2 and, hence, the screen-grid potential, may be established at the desired magnitude.

A second embodiment represented in Fig. 2 may comprise a tetrode 20 having its plate 22 coupled via load resistor 24 to a source of B+ potential and its cathode 26 coupled to a ground source of constant potential via cathode resistor 28, and a regulator triode 30 having its plate 32 coupled to the aforesaid source of B+ potential, its cathode 34 coupled directly'to the screen-grid 36 of tetrode 20, and its control grid 38 coupled to a source of reference potential established by resistors 40 and 42 coupled as a voltage divider between the source of B+ potential and the cathode 26 of tetrode 20. A bypass capacitor 44 is coupled in parallel with resistor 42 of the voltage divider to provide a low-impedance path between cathode 26 and control grid 28 for fluctuating potentials which, for example, may be representative of an A.-C. input signal e applied to the control grid 46 of tetrode 20. An output signal e may be derived from a point between tetrode plate 22 and load resistor 24.

The operation of the embodiment of Fig. 2 is similar to that of Fig. 1 for D-.-C. inputsi'gnals or A.-C. input signals of such low frequency that bypass capacitor 44 becomes a high-impedance. Thus, increasing space current in tetrode 20 results'in more electrons being collected by screen-grid 36, thereby tending. to lower the potential of cathode 34 of triode regulator tube 30'. The tendency of the cathode 34 to become more negative is counteracted, as previously explained,-by increasing space current through triode 30, thereby preventing, or at least minimizing, any change in screen potential.

Inasmuch as cathode resistor 28 of tetrode 20 is unbypassed, the potential level on cathode 26 will follow any potential fluctuations of input signal e Therefore, where the frequency'of e is sufficiently high, the corresponding potential variations on cathode 26 are passed via capacitor 44 with little,.. if any, attentuation, thereby causing the reference potential of the regulator triode 30 to follow exactly the potential changes occurring at cathode 26. As explained previously, the cathode follower action of regulator triode 30 means that the potential on its cathode 34 and, hence, the potential on screen-grid 36 will change in phase with, and to an extent proportional to, the fluctuations on control grid 38. In this manner the potential difference existing between cathode 26 and screengrid 36 of tetrode 20 may be rendered substantially invariable.

On the other hand, when the input signal 2 is a slowly changing D.-C. potential or an A.-C. voltage having a sufiiciently low frequency, the attenuation occurring in the R-C coupling network comprised of resistor 42 and capacitor 44 becomes very large and, as a result, only a fraction of the potential change occurring on cathode 26 appears at control grid 38 of regulator triode 30. Ac-

cordingly, the principal compensating action of regulator' triode 30 is similar to that explained above in connection with the function of triode regulator 2 of the embodiment of Fig. 1. It should be understood, of course, that in many conditions of operation of the embodiment of Fig. 2, the effect of a changing reference potential on control grid 38 and the effect of increased screen current attrib utable to changes in average D.C. level in tetrode 20 will combine as may be required to stabilize eflectively the cathode-screen potential difference. The average DC. potential of cathode 26 will change, of course, in response to fluctuations in the magnitude of a D.-C. input signal or in the amplitude of an A.-C. input signal. Notwithstanding the character of the input signal e the regulator'triode 30 in the circuit of screen-grid 36 effectively renders the cathode-screen potential difference invariable.

A third exemplary embodiment of the invention represented in Fig. 3 comprises tetrodes 50 and 52 coupled in parallel between a ground source of constant potential and a common source of B+ potential, thereby forming an electron-tube circuit arranged for push-pull operation. The two halves of the primary winding 54 of output transformer 56 comprise the respective plate loads for tetrodes 6 use as a power amplifier at audio frequencies, the come ponents listed below were found-to give excellent results:

Inasmuch as tetrode 52 and. the components of it's screen-potentialregulator circuit are" duplicates of: those set-'forth above, they have been omitted from. the list.

It should be understood that many variations may be made in the structure of the aforedescribedi embodiments of the invention. For example, any screen-grid, spacedischarge device may be used in lieu of any of those represented in the drawings. Thus, pentode 1 of Fig. 1

could be a tetrode or any other multigrid device wherein 50 and 52. The B+ source of plate potential is coupled to the center tap of primary winding 54. An output signal e may be derived from the terminals of the secondary winding 58 of output transformer 56.

Inasmuch as the respective screen-grid circuits of tetrodes 50 and 52 are the same as the one described above in connection with the embodiment of Fig. 2, the same reference numerals are applied to similar components of both figures.

The operation of the screen-grid potential regulator circuit is essentially the same as that of the embodiment of Fig. 2. Accordingly, a detailed description thereof is unnecessary.

When utilized in a push-pull amplifier circuit such as that represented in Fig. 3, it is possible to obtain an output signal of considerably more power and less distortion than that ordinarily provided by amplifiers heretofore known. In an actual embodiment constructed for one of the elements fulfills the functional requirements of a screen-grid. The regulator triode could be any spacedischarge device having an electrode for controlling the flow of space current. Furthermore, it is obvious that voltage regulator tubes of the gas-discharge type may be utilized to perform their conventional function in maintaining constant any bias or reference potential present in the circuit. Whether bypass capacitors are utilized will depend upon the desired functional characteristics of any particular circuit which may be constructed in accordance with this invention.

The structure represented in the accompanying drawings and described above is intended merely to facilitate the practice of the invention by persons skilled in the art. The scope of the invention is represented more accurately in the following claims.

I claim:

1. An electron-tube circuit for maintaining a'constant potential difference between the cathode and screen-grid of a first space-discharge device in a circuit including an impedance between the cathode and a fixed potential to provide space-discharge therethrough comprising: a second space-discharge device having at least an anode coupled to an anode supply voltage, a cathode, and control grid; means coupling the cathode of the said second space-discharge device directly to the said screen-grid; and means coupling the said control grid to the cathode of the said first space-discharge device through a resistance and a capacitance in parallel, said control grid having a bias voltage thereon whereby said control grid bias is responsive to voltage variations of said first spacedischarge device cathode for controlling the screen-grid voltage.

2. An electron conduction circuit wherein amplitude distortions and power-output losses are minimized comprising: a space-discharge device for establishing a stream of electrons in space, the said device including an electron-collecting element disposed at an intermediate location in the said electron stream between electrondischarging and receiving elements, said electron-discharging element being coupled through a resistance to a fixed potential; a source of direct-current potential; first means coupling the said discharge device to the said source of direct-current potential such that the said electron stream is established; a source of reference potential; and second space-discharge means having an electron-discharging element directly coupling the said electron-collecting element, having an electron-receiving element coupling said source of direct-current, and having an electron control element coupling said reference potential and coupling said electron-discharging element of said first space-discharge device through a resistance and a capacitance in parallel for maintaining a fixed potential difference between the said electroncollecting element and the said 'source'of reference potential notwithstanding large fluctuations in the volume of electrons collected by the said electron-collecting element.

3. An electron-tube circuit for controlling the screen grid voltage of an electron tube therein comprising: a first space-discharge device having at least an anode, a cathode, and i a screen-grid; a second space-discharge device having at least an anode, a cathode, and a control grid; anode potentials coupled to the anodes of said first and second space-discharge devices and a fixed potential coupled through a resistance to the cathode ofsaid first space-discharge device for providing conduction therethough; means coupling the cathode of said second space-discharge device to said screen-grid for maintaining the potential thereon equal; means coupledvto said control grid of said second space-dis'- voltage on said control grid in accordance with said first space-discharge. device cathode potential whereby the potential diflference existing between the cathode and screen-grid of said first space-discharge device is substantially invariable. t

' References Cited in the tile of this patent UNITED STATES. PATENTS Bernard Oct. 30, 1951 

